With advancing age, several characteristic alterations occur in the cardiovascular system. One of the most commonly described changes in senescence is a moderate degree of myocardial hypertrophy in both animals and in man. Other alterations seen in the senescent hearts are a prolonged time course of relaxation (Weisfeldt et al., Am. J. Physiol. 220: 1921-1927, 1971; Lakatta et al., J. Clin. Invest. 55: 61-68, 1975), a diminished rate of sarcoplasmic reticulum calcium accumulation (Froehlich et al., J. Mol. Cell. Cardiol. 10: 427-438, 1978) and an increased active dynamic stiffness in senescent compared with mature heart muscle (Spurgeon et al., Am. J. Physiol. 232(4): H373-H380, 1977) and intact heart (Templeton et al., Circ. Res. 44: 189-194, 1979). Although the extent to which these functional alterations seen with senescence are related to the concurrent hypertrophy is unknown, a relationship is suggested since prolonged contraction duration, diminished sarcoplasmic reticulum function, and increased stiffness have been observed in experimental models of hypertrophy.